Unlock Characters & Modes
Do the following task to unlock the following characters and modes:

Black Panther - Collect Black Panther Action Figures throughout various acts.
Blade - Play Crane Game in Murder World.
Daredevil - Collect Daredevil Action Figures throughout various acts.
Dr. Strange - Rescue him from Mandrin in Act 1.
Ghostrider - Rescue him from Mephisto in Act 2.
Hard Mode - Complete the game on easy or normal mode.
Nick Fury - Complete the game on easy or normal mode.
Silver Surfer - Unlock by beating ALL COMIC missions in the simulator.

Unlock Costumes
These are just how you unlock all the character's alternate costumes:

2nd Costume - Defeat 30 enemies with that character
3rd Costume - Defeat 175 enemies with that character
4th Costume - Earn it after beating that characters comic book mission

Bonus Game Modes
Finish the game once (Normal difficulty) to unlock the following modes:

Hardcore Heroes (Heroes cannot be revived once they have been killed - can import save data to boost initial stats).
Hardcore Squad (Initial team selection fixed for entire game; heroes cannot be revived once they have been killed; can import save data).
Uber Hero (Play with a single hero, who's level is raised to 99 and all powers and outfits maxed).